For the purpose of providing a brief background of this kind of a voice signal and dial pulse transmitting circuit, which will enhance an understanding of the operation of the circuit of the present invention, reference is made to FIG. 1.
Shown in FIG. 1 is a conventional circuit for transmitting voice signals and sending dial pulses between telephone network lines, including a rectifier circuit 1, dial pulse transmission circuit 2, and telephone voice transmission circuit 3. The circuits 1, 2 and 3 are separately provided between telephone network lines L1 and L2 and are connected to the following circuit (not shown) via a voice transformer 4.
The dial pulse transmission circuit 2 has a photo-transistor PT having its collector and emitter connected to the telephone network lines L1 and L2, respectively, and a resistor R1 connected to the primary winding 4a of the voice transformer 4 via a two-way relay SR and a capacitor C so as to form a closed circuit when dialing is performed, the photo-transistor PT is made conductive and nonconductive (or turned ON and OFF), responding to instructions provided from a related control unit (which is well known in the art and not shown in FIG. 1). The photo-transistor transmits its signals in a known manner as dial signals between the telephone network lines.
The voice transmission circuit 3 has a transistor Tr with its collector and emitter connected to the telephone network lines L1 and L2, respectively. The transistor Tr is connected, at its base, to one terminal of a varistor VA, which, in turn, is connected at its other terminal to a junction of the primary winding 4a of the voice transformer 4 and a capacitor C.
In the above circuit, during a call, the two-way relay SR is in contact with a first contact a so as to render the transistor TR operative and to render the photo-transistor PT nonconductive. During dialing, the two-way relay SR is in contact with another, second, contact b so as to allow the photo-transistor PT to turn conductive and nonconductive, or ON and OFF. Responding to the ON-OFF operation of the photo-transistor PT, dial pulses are provided and transmitted to the telephone network line.
The known circuit described above renders the photo-transistor PT useless while the transistor Tr is utilized during a call and the transistor Tr useless while the photo-transistor PT is utilized during dialing. Thus, it can be said that the prior art circuit is provided with two separate circuit parts which perform almost the same function. This results not only in a cost increase but also in a complicated circuit structure.